The present invention relates to the field of switched capacitor filters design and implementation. More specifically, the present invention relates to a common mode feedback circuit using switched capacitors.
In many filter designs, a differential amplifier can be used to improve the signal to noise ratio (SNR) and to improve immunity to noise coupling into the circuit. One of the drawbacks to differential amplifiers is the need for a common-mode feedback circuit. In order for the common-mode loop to be stable, the poles introduced in the common-mode loop need to be well above those of the differential loop. This can require large amounts of power and design complexity. A solution to this problem is to use two single-ended amplifiers with their positive inputs tied to the desired common-mode voltage. Generally, this circuit has the same properties as the fully differential design with respect to the SNR and noise immunity improvement without having to use a common-mode feedback structure.
One of the limitations on power consumption in switched capacitor circuits is the need to quickly charge switched capacitors to a desired accuracy in a limited period of time (e.g., half a clock cycle). Since the transfer function in a switched capacitor filter is based on capacitor ratios, the capacitors could be made as small as the photo-lithography and parasitic effects will permit without altering the transfer function. The true limit on capacitor sizes however, arises from the noise resulting from the resistive switch that is sampled onto the capacitor along with the desired signal. This noise power is inversely proportional to the switched capacitor value, hence for a required noise performance, a minimum capacitor size is indicated. This will also determine the capacitive load on the amplifiers in the switched-capacitor filter. For a given clock frequency and charge accuracy, the power required by the operational amplifiers (opamps) approximately scales with the square of the capacitor size. One way to reduce the capacitor size without degrading the noise performance is to use cross-sampling. Cross-sampling is a method where the switched capacitor is referenced to the inverse of the signal rather than an analog ground, effectively doubling the amount of charge transferred by the capacitor allowing the same transfer function to be realized with one-half the capacitance.
However, there is a problem in using the pseudo-differential and the cross-sampling methods in switched capacitor filter design because there is no common-mode information being transferred from the input to the output of the opamp. The common mode transfer function is not defined since there is no path for the input common mode to the output. In addition, there is no feedback of the output common mode voltage. In the presence of offsets, the output of the filter will saturate and the filter will no longer perform its intended function.